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On infrared images taken with the NEOWISE satellite on Jan. 3, 2021 a comet was discovered near the border of the constellations Puppis/Centaurus. Comet C/2021 A2 (NEOWISE) was then observed by CCD and visual observers. It displayed a very diffuse coma of diameter about 2' and total magnitude 12.5, but no tail (CBET 4908). The comet will reach perihelion on Jan. 22, 2021 at the solar distance of 1.41 AU, expected to reach magnitude 11.0. Thereafter it approaches Earth with a minimum distance of 0.41 AU on Feb. 3. It should be brighter than 16 mag until the start of May. During this interval it moves through the constellations Centaurus, Pyxis, Puppis, Monoceros, Orion, Taurus towards the northern border of Auriga. Mid-European observers will see it rise above the southern horizon during the last week of January around midnight. Thereafter it will be an evening object with increasing altitudes.
The comet experienced a high level of activity throughout the apparition. At magnitude 12.5 at the start of January it peaked at magnitude 10.6 at the end of January 2021. Thereafter it faded quickly, raching 16 mag by the end of March. Based on 76 observations from 28 observers the appropriate brightness parameters are
m0 = 6.7 mag / n = 14Total Brightness and Coma Diameter
Between the opening of January and Feb. 10 the coma diameter increased from 1' (45.000 km) to the maximum of 4' (110.000 km). Thereafter it decreased, reaching 0.5' (30.000 km) by the end of March. Between the start and the end of January the degree of condensation increased from DC 2 to DC 4, decreasing to DC 1-2 until the start of March. No tail was observed.
Andreas Kammerer